1. Field of the Invention
The present invention relates to applying power to a daughtercard to maintain a low impedance power supply path and to accurately control the impedance of lines carrying signals. More particularly, the present invention relates to applying such power to daughtercards utilized in a telephone signal distribution bank.
2. Description of the Background Art
A signal distribution bank used by telephone companies includes shelves or slots on a backplane for connecting daughtercards. The daughtercards include "multiplexer" cards for switching signals provided to the distribution bank, and "distribution" cards, for transferring signals from the distribution bank to remote users or subscribers. With such signal distribution banks, telephone companies can start with only a few daughtercard slots occupied and later fill in the remainder. One such signal distribution bank is the Litespan.RTM. LS2012 manufactured by DSC Communications Corporation, Plano Tex.
Power is typically supplied to the daughtercard from power supplies provided on the backplane. To minimize power supply noise, it is desirable to provide a low impedance path from the power supply of the backplane to components of the daughtercard. Typically, a limited number of pins on a daughtercard are utilized to provide power from the backplane because it is desirable to utilize as many pins as possible to carry signals to and from the backplane. With a limited number of power supply pins, components located farther from a power supply pins will have a longer line length carrying power, the longer line length increasing impedance. With increasing line impedance, power supply voltage provided to components on the daughtercard will decrease. Thresholds of components receiving power from longer lines with less voltage will then be reached later than thresholds of components with shorter power supply lines and higher voltages. With thresholds reached at different times, data and clock signal transitions can occur in an asynchronous manner potentially causing errors. Further, with higher impedance power supply lines, when data and clock signals transition, greater noise will be created on the power supply lines which can affect other components on the daughtercard, as well as the power supply signals provided from the backplane itself.
To limit noise created in high frequency signal lines separate from the power supply lines, it is further desirable to have the lines carrying signals between the backplane and the daughtercard have a uniform impedance. With signals switching at a high frequency, signal lines on the daughtercard and backplane are typically provided with line widths and separation from a ground plane set to maintain line impedance at a desired value. With pins connecting the backplane and daughtercard without a ground plane to control impedance, an undesirable impedance mismatch may occur creating signal noise.